Sometimes your wonderful Amazon Kindle may get hung and need a little help. How to go about issuing a hard or soft reset depends on which version of the Kindle you have.
Kindle 1 (K1)
Soft Reset: Hold down Alt-Shift-R
Hard Reset: Remove the back cover and insert a paperclip in the small hole in the back. You may have to do this for 10 seconds or more and the Kindle should be plugged in during this time.
Kindle 2 (K2)
Hold the slider switch to the right until the screen starts flashing. This can take 10 seconds or longer. The Kindle should then begin its usual start-up operations. Be patient! I've seen the Kindle screen go completely blank and then suddenly start booting 60 seconds later.